Swift observes a new outburst from the Supergiant Fast X-ray Transient SAX J1818.6-1703

ATel #2191; P. Romano (INAF-IASF Palermo), S. Barthelmy (GSFC), L. Sidoli (INAF-IASF Milano), S. Vercellone (INAF-IASF Palermo), D. N. Burrows (PSU), M. M.Chester (PSU), P. Esposito (INAF-IASF Milano), P. A. Evans (U. Leicester), N. Gehrels (GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), V. La Parola (INAF-IASF Palermo)
on 5 Sep 2009; 19:00 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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The Swift/BAT triggered on a new outburst from the Supergiant Fast X-ray Transient SAX J1818.6-1703 on 2009 September 5 at 11:15:15. Using the data set from T-239 to T+963 s, the mask-weighted light curve shows weak on-going emission at the start of the data at T-240 sec with a slow rise to a peak (of 0.06 ph/cm2/s in the 15-350 keV band) somewhere around T+200 s, and then slowly decreasing out past the end of the collected event-by-event data at T+963 s. We also have 5-min sampled DPH data (in the 15-195 keV band) from T+500 s out to T+2100 s. That mask-weighted lightcurve shows another slow increase to 0.04 ph/cm2/s around T+1800 s with ongoing emission past the end of data at T+2100 s.

The BAT time-averaged spectrum from T+0 to T+320 s is best fit by a simple power-law model. The power-law index of the time-averaged spectrum is 2.98 +- 0.50. The fluence in the 15-150 keV band is (6.2 +- 1.5)E-7 erg/cm2. The 1-s peak photon flux measured from T+0.00 s in the 15-150 keV band is 0.045 +- 0.008 ph/cm2/s. All the quoted errors are at the 90% confidence level.

Swift performed an immediate slew, so that the XRT began observing the field at 11:21:44.0 UT, 389.0 seconds after the BAT trigger. The Swift/XRT light curve of the first orbit of data (up to ~T+2130 s) shows several flares of increasing intensity; during the last one, the source reached ~15 counts/s. This peak roughly coincides with the second rise observed by BAT. The Swift/XRT spectrum from WT data (766-2025s since the trigger, with a gap to include PC data) can be fit with an absorbed power law with a photon index of 1.6+/-0.2 and an absorbing column density of NH=(8+/-1)E+22 cm-2. The mean 0.3-10 keV observed (unabsorbed) WT flux is 8E-10 (2E-9) erg/cm2/s.

The PC data (395-2131 s since the BAT trigger) are affected by contamination from single-reflection of stray light from outside the XRT field of view. Fitting the PC data with an absorbed power law we obtain a photon index of 1.1+/-0.4, NH=(8+/-2)E+22 cm-2 and an observed (unabsorbed) flux of 1E-9 (2E-9) erg/cm2/s. An unabsorbed flux of 2E-9 translates into a luminosity of 1.5E+36 erg/s (assuming the optical counterpart distance of 2.5 kpc).

Previously, Swift observed flares from this source on 2009 May 6 (Romano et al,ATel #2044, Sidoli et al. 2009,MNRAS,in press,arXiv:0907.4041), 2008 March 15 (Barthelmy et al., GCN #7419), and 2007 October 16. The SAX J1818.6-1703 light curve from the BAT hard X-ray transient monitor (15-50 keV) can be found at http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/SAXJ1818.6-1703/
The characteristics of this burst are consistent with the ones observed in the 2009 May 6 flare.